Structural and thermodynamic change upon the antigen binding of human thrombopoietin neutralizing IgG TN1

Arai, Shigeki; Tamada, Taro; Honjo, Eijiro; Maeda, Yoshitake*; Kuroki, Ryota

The mouse antibody TN1 recognizes the human thrombopoietin (hTPO) that primarily stimulates megakaryocytopoiesis and platelet production. By the structural comparison between TN1-Fab itselt (hTPO unbound form) and hTPO bound form of TN1-Fab (PDB id 1V7M and 1V7N), we found that the CDR of TN1-Fab need not accompany the large conformational changes of upon TPO recognition. Moreover, the isothermal titration calorimetry showed that the conformational entropy change upon the hTPO binding to the TN1-Fab was 446.4 kJ/mol/K, corresponding to 2,920 burying upon hTPO binding. This change of accessible surface area is larger than that of the previous result (1,580 ) estimated from the crystal structure of hTPO / TN1-Fab complex. Since the CDR structure of TN1-Fab did not change, the change in surface area may be from the conformational change of hTPO upon the binding to Fab.